This invention generally relates to associative thickeners. In particular, the present invention relates to associative thickeners having enhanced viscosity stabilization.
In the formulation of aqueous latex paints, a balance between high and low shear viscosities is sought to provide satisfactory applications properties, such as by spraying or brushing, leveling of the applied wet paint film, and minimal sagging of the applied wet paint film. Aqueous latex paints are commonly formulated with various viscosity modifying additives, often referred to as flow modifiers, viscosity modifiers, rheology modifiers, or thickeners, to balance the high and low shear flow properties of the aqueous latex paint. Examples of viscosity modifying additives include cellulosic materials, such as hydroxyethyl cellulose, alkali soluble emulsions, associative thickeners, and ionic polymers. Typically, blends of viscosity-modifying additives are employed to obtain the appropriate balance of high and low shear viscosities.
Associative thickeners operate to thicken aqueous systems to which they are added by the non-specific associations, such as adsorption on dispersed phase surfaces and aggregation in solution akin to micellization, between the hydrophobic groups on the thickener molecules and moieties on the other components in the system, similar to the non-specific associations of conventional surfactants. Because associative thickeners comprise at least two hydrophobic groups, a network of associations is established. This network increases the viscosity of the aqueous composition.
A problem that may occur in paints containing associative thickeners is a drop in mid-shear (Krebs Stormer) viscosity when colorants that contain high levels of surfactant are added. Mid-shear viscosity is the viscosity exhibited when the applied shear rate is in the range of 10 to 100 sec−1. This drop in mid shear viscosity is especially problematic when the paint will be tinted to a deep tone because a high level of surfactant generally accompanies the colorant.
Generally, it is possible to formulate a light tint base at a high enough mid-shear viscosity that colorants added to it will not depress the viscosity to an unacceptable degree. In some instances, combinations of associative thickeners have been found to be less sensitive to colorant addition than the individual thickeners alone. Combinations of two or more associative thickeners are commonly used in paints to obtain the desired balance of high and low shear rate viscosities. These common combinations include a pseudoplastic associative thickener to control the low to mid shear viscosity and a more Newtonian associative thickener to control the high shear viscosity. In attempting to solve the problem of mid-shear viscosity loss upon tinting, U.S. Pat. No. 6,337,366 describes the use of a mixture of a select associative thickener component and a select surfactant component. The ratio of the molecular weight of the surfactant to the molecular weight of the associative thickener is relatively large in comparison to similar components in conventional aqueous systems. However, the required relatively high concentration of surfactant component tends to diminish sag resistance of the paint.